1. Field of the Invention
The present invention relates generally to a support structure for tubing within a steam generator. More particularly, the invention relates to an array grid that provides lateral support for steam generator tubing while minimizing the risk of attack by stress corrosion cracking to the steam generator tubes.
2. Description of Related Art
Steam generators are used in nuclear power plants to transfer the fission-generated heat from the plant""s primary coolant system to the plant""s secondary steam system. Typically, the primary coolant flows through tubes within the steam generator, while secondary water surrounds the outer portion of the tubes. To ensure sufficient heat transfer from the primary coolant to the secondary water, the steam generator tubes must be sufficiently thin, be constructed of a material having a relatively high heat transfer coefficient, and have sufficient heat transfer area. Additionally, the material of construction must be able to withstand the high level of differential pressure developed across the tube from the primary plant side to the secondary steam plant side. Meeting these design requirements requires sufficiently long runs of tubing within the steam generator. Thus, there is a need to provide lateral support for the tubing at various points along its traversal in and through the steam generator.
Lateral support for steam generator tubing is conventionally provided by a so-called xe2x80x9ceggcratexe2x80x9d grid array. A conventional grid array consists of a plurality of spaced apart interlocking bars or strips. The steam generator tubes extend through the spacing between the strips and physically contact side portions of the strips.
One problem long associated with nuclear steam generators has been the insidious attacking of the tubes by various corrosion phenomena, including wastage, pitting, general intergranular attack, and of course, stress corrosion cracking. Stress corrosion cracking is a complex phenomenon, the mechanism for which is not fully understood. Nonetheless, it is generally believed that it involves a local galvanic attack or a reduction in surface energy by chemisorption, or a combination of both. Additionally, it is believed that there are environmental effects associated with stress corrosion cracking, the two most important of which are the tensile stress level and the corrosion media""s chemistry.
Steam generator tubes are under constant tensile stress during operation, since the primary system pressure within the tubes is significantly higher than the steam plant pressure surrounding the tubes. Moreover, the secondary plant water within the steam generator is usually maintained at a relatively high pH ( greater than 10.0) to alleviate other types of corrosive attack. Yet, because of the areas of contact between the steam generator tubes and the conventional eggcrate grid array, accumulation of particulate matter (colloquially referred to in the art as xe2x80x9csludgexe2x80x9d) can occur in these areas of contact. When fluid within the steam generator is boiled, in order to produce steam, a portion of the non-volatile chemicals are drawn into the accumulated sludge. As a result, the accumulated sludge concentrates chemicals in the area, increasing the localized pH, and thereby enhancing the likelihood of stress corrosion cracking. Once cracking begins, if it reaches a certain xe2x80x9ccritical length,xe2x80x9d it can result in a xe2x80x9cfish mouthxe2x80x9d rupture of the tube.
Thus, there is a need to provide a grid array that laterally supports tubes in a steam generator, while simultaneously minimizing sludge accumulation at the contact points between the grid array and the tubes.
It is therefore a principal object of the present invention to provide a steam generator tube support grid array that provides sufficient lateral support for the tubes, while at the same time reduces the likelihood of attack to the tubes by stress corrosion cracking.
In one aspect of the present invention a support structure for tubes within a steam generator comprises a plurality of first elongate strips including a plurality of first slots, and a plurality of second elongate strips. The plurality of first slots each have an open end positioned at a top end of the first elongate strips and a closed end positioned proximate a center portion of the first elongate strips. A width of the closed end of the first slots being greater than a width of the open end thereof. The plurality of second elongate strips are each received into one of the plurality of first slots.
In another aspect of the present invention, a support grid for tubes within a steam generator comprises a plurality of first and second elongate strips. The plurality of first elongate strips each have a cross section including a first protuberant portion, and each include a plurality of first slots arranged along a length thereof. The plurality of second elongate strips have a cross section including a second protuberant portion, and each of the second elongate strips is received into one of the plurality of first slots.
In still another aspect of the present invention, a lateral support grid array for steam generator tubes comprises a plurality of first and second elongate strips. The first elongate strips include a plurality of first slots each having an open end positioned at a top portion of the first elongate strips and a closed end positioned proximate a center portion of the first elongate strips. A portion of the slots, intermediate the open end and the closed end, has a width greater than a width of the open end and the closed end thereof. Each of the plurality of second elongate strips is received into one of the plurality of first slots.
In yet a further aspect of the present invention, a tube support grid for providing lateral support for tubes within a steam generator comprises a plurality of first, second, third, and fourth elongate strips. The plurality of first elongate strips each have a cross section including at least one protuberant portion, and each include a plurality of first and second slots arranged in alternating fashion along a bottom portion thereof. The first elongate strips extend in a first direction in the support grid array. The plurality of second elongate strips each have a cross section including at least one protuberant portion, and each include a plurality of third and fourth slots arranged in alternating fashion along a top and bottom portion thereof, respectively. Each of the third slots is interlocked with one of the first slots, whereby each of the second elongate strips extend in a second direction, different from the first direction, in the support grid array. The plurality of third elongate strips each have a cross section including at least one protuberant portion, and each include a plurality of fifth and sixth slots arranged in alternating fashion along a top portion thereof. Each of the fifth slots is interlocked with one of the second slots, whereby each of the third elongate strips extend in the second direction, parallel to the second elongate strips. The plurality of fourth elongate strips each have a cross section including at least one protuberant portion, and each include a plurality of seventh and eighth slots arranged in alternating fashion along a top and bottom portion thereof, respectively. Each of the seventh slots is interlocked with one of the fourth slots and each of the eighth slots is interlocked with one of the sixth slots, whereby each of the fourth elongate strips extend in the first direction, parallel to the first elongate strips.
In yet still another aspect of the present invention, a tube support grid for tubes within a steam generator comprises a plurality of first and second elongate strips. The first elongate strips have a cross section including a first protuberant portion positioned such that the first elongate strips have a substantially T-shaped cross section. The second elongate strips have a cross section including a second protuberant portion positioned such that the second elongate strips have a substantially T-shaped cross section. A plurality of slots are arranged along a length of the first elongate strips. Each of the first slots includes an open portion and a closed portion, the open portion having a width dimensioned so as to receive the second protuberant portions. Each of the second elongate strips is received into one of the plurality of slots.